The design and testing requirements are contained in the CENELEC and IEC Standard IEC 60079‐1. Either tapered (NPT) or parallel (straight or metric) threads are acceptable. The conditions are: The Ex d enclosure must be certified. When rainstorms hit or high-pressure washdowns begin, this rating becomes your best friend: The. Safely conduct, connect and distribute energy in hazardous areas with R. Either tapered. The purpose of this document is to provide general information on the definitions of NEMA Enclosure Types to architects, engineers, installers, inspectors and other interested parties.